<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">我是一个按钮</button>    
    <script>
        // getElementById()可以通过id名获得元素的对象
        var btn = document.getElementById("btn");
        // innerHTML()可以修改内部的HTML代码
        btn.innerHTML = "I am a btn";
    </script>
</body>
</html>